Large public displays such as digital signage displays and information displays are commonly used outdoors. The viewing-side surface of an image display panel such as a liquid crystal panel or an organic electroluminescent (EL) panel used outdoors is provided with a front protective plate such as a transparent resin plate or a glass plate, in order to prevent damage to an image display panel by impacts from the outer surface and provide protection against wind and rain and dust from the outside. Generally, the viewing-side surface of a front protective plate is provided with an antireflection layer in order to suppress a reduction in visibility due to reflection of external light such as sunlight (e.g., JP-A-11-258592).
When a front protective plate is disposed on the viewing-side of an image display panel, an air gap structure (hollow structure) is employed in which an air gap is provided between a panel and a front protective plate for protecting the surface of the panel. In the air gap portion, the refractive index of air is about 1, whereas the refractive index of a plastic material or glass material that forms a polarizing plate or a front protective plate on the surface of an image display panel is about 1.5, and therefore when the air gap structure is employed, reflection or refraction of light at the interface increases. Accordingly, an image display device having a front protective plate provided on the surface of a panel has a problem that diffusion/scattering of video light emitted from an image display panel, and reflection of external light, such as sunlight easily occur, leading to a reduction in visibility (especially contrast).
In a small display mounted on a mobile phone, a smart phones or the like, an “interlayer filling structure” is employed in which a gap between an image display panel and a front protective plate is filled with a transparent resin material or pressure sensitive adhesive having a refractive index close to a refractive index of glass, resin or the like, so that reflection, scattering or the like of light due to a refractive index difference in the gap portion is reduced. On the other hand, in a large display such as a public display, it is not easy to fill a gap portion between an image display panel and a front protective plate with a resin material. When the gap portion is filled with a resin material, there is a problem that the panel temperature increases because heat of the front protective plate is easily transmitted to the panel.
JP-A-11-258592 describes a configuration in which circularly-polarizing plates are disposed above and below the gap portion, respectively, in order to improve visibility in an image display device having an air gap structure. In this configuration, the contrast can be improved by selectively emitting video light from an image display panel to the viewing-side, and suppressing reflection of external light to the viewing-side.
In addition to the problem of reduced visibility due to external light such as sunlight, a large image display device to be used outdoors has a problem that the panel temperature is caused to increase by external light. When the panel temperature increases, a display defect may occur due to degradation of an element, and an orientation disorder of liquid crystal. A main cause of panel temperature increase is near infrared rays entering thereto. By providing an infrared shielding element on a surface of a front protective plate or a panel, incidence of near infrared ray is reduced to suppress an increase in panel temperature. For example, WO 2010/109723 discloses a structure in which an anti-reflection structure is provided on viewing-side of a front protective plate, and a circularly-polarizing plate and an infrared shielding layer composed of an infrared ray absorbing material are provided on the display panel-side of the front protective plate. According to this structure, an increase in panel temperature can be suppressed by shielding of infrared ray while reflection of visible light is suppressed.